The goal is to provide a ubiquitous, flexible, comprehensive-as-possible emulator that will appear in as many browsers as possible without installing a plugin or runtime. While a number of emulation solutions exist that allow much of what is wanted, they nearly all require plugins and most are directed towards a single machine or small sets of machines.
MESS and MAME were started over a decade ago to provide ubiquitous, universal emulation of arcade/gaming machines (MAME) and general computer hardware (MESS). While specific emulation implementations exist that do specific machines better than MAME/MESS, no other project has the comprehensiveness and modularity. Modifications are consistently coming in, and emulation breadth and quality increases over time. In the case of MAME, pages exist listing machines it does not emulate.
The dream/goal is to provide access to computer software and artwork that would otherwise require the user to have the original hardware and software at hand to bring into existence. While nothing beats having vintage, well-maintained computer hardware to show what software "was", it requires advocacy and often physical presence to do so. Games like Pac-Man' or Super Mario have been re-done many times and provided in all manner of online and offline presentations... not so much examples of Wordperfect, Peachtree Accounting, or the Atari TOS. By providing this method of calling in software, historians and academics and the merely curious can get near-instantaneous access to the gist of these early programming works. As a side benefit, people with collections of old software will be more inclined to share or donate their piles of materials knowing that universal access will come shortly after.